1. Field of the Invention
The present invention relates to a method of and an apparatus for forming, a thin film on a surface of a semiconductor substrate and, more particularly, to a method of and an apparatus for forming a thin film in which an interface structure between the semiconductor substrate and the thin film is reliably controlled.
2. Description of the Background Art
Characteristics of electronics devices are readily and adversely affected by contaminants introduced on purpose or by accident in the course of the manufacturing process. In order to avoid any introduction of contaminants into the electronics products as much as possible, it is necessary to keep the whole manufacturing environment at a maximum possible degree of cleanliness. In this connection, highly advanced cleaning and purifying technologies are employed to produce desired starting materials and processing environments for the manufacture of the devices.
The manufacturing process for semiconductor devices is roughly divided into the steps of forming thin films and a circuit pattern. The process for forming thin films on the semiconductor substrate is further divided into many subprocesses depending on the materials of which the thin films are made and techniques to be used. Various cleaning technologies for each of the subprocesses or to be shared by some of the subprocesses have been developed into practical use. Important and essential to all these subprocess is the pretreatment of the semiconductor substrate which is performed prior to forming the thin films on the substrate.
In the pretreatment process, the semiconductor substrate is cleaned with water, acid or alkalis, and it is subjected to chemical oxidation or a treatment with dilute hydro fluoric acid, for the removal of grease, heavy metals, naturally grown oxide film and the like. These solution cleaning techniques are widely used in the industry but their decisive disadvantage is that the cleaned semiconductor substrate is unavoidably exposed to the surrounding air before it is coated with the thin film in a subsequent process. The exposure to the air causes a thin oxide film to grow on the substrate surface, especially when the substrate contains an active semiconductor substance or it has exposed metal portions thereon. For this reason, the substrate cleaning with solutions falls short of complete cleaning of the substrate surface although it is effective to remove heavy metals and organic contaminants.
The growth of a natural oxide film on the semiconductor substrate has an adverse effect on the quality of various films to be formed on the substrate in a subsequent step. The thin films provided on the semiconductor substrate include epitaxially grown layers, layers of high melting point metals or polysilicide layers, electrical interconnection layers, and ultra-thin insulating layers. The formation of these layers on the semiconductor substrate is growing in importance as the degree of integration of semiconductor devices advances. Therefore, in order to remove harmful contaminants in the film and also improve the quality of the thin film, an effective method of forming a thin film on the semiconductor substrate with a well-controlled interface between them has been long been waited for.
FIG. 15 is a sectional view of a conventional sputtering apparatus capable of processing a plurality of semiconductors at the same time. Referring to FIG. 15, the conventional sputtering apparatus comprises a chamber 20. The chamber 20 is divided into a loader chamber 22, an etching chamber 23, a deposition chamber 24 and an unloader chamber 25 by lock valves 21a, 21b and 21c.
The loader chamber 22 comprises a nitrogen gas inlet 26a and an outlet 9a. In the loader chamber 22, a carrier system 27a such as belt conveyer is provided.
The etching chamber 23 comprises an Argon gas inlet 28a for introducing Argon gas and an outlet 9b. In the etching chamber 23, a carrier system 27b is provided. The etching chamber 23 comprises a substrate supporting plate 29a. A tray 30 is placed on the substrate supporting plate 29a and a semiconductor substrate such as a silicon substrate 4 is put on the tray 30. A high frequency power supply 31 is connected to the silicon substrate through a matching circuit 32. The substrate supporting plate 29a is insulated from the chamber 20 by an insulating portion 33a.
The deposition chamber 24 comprises an Argon gas inlet 28b for introducing Argon gas and an outlet 9c. A carrier system 27c is provided in the deposition chamber 24. The deposition chamber 24 comprises a substrate supporting plate 29b. In addition, the deposition chamber 24 comprises a target 34. The DC power supply 35 is connected to the target 34 and a high voltage is applied between the target 34 and the substrate supporting plate 29b. The substrate supporting plate 29b is insulated from the chamber 20 by an insulating portion 33b and the target 34 is insulated from the chamber 20 by the insulating portion 33b.
The unloader chamber 25 comprises a nitrogen gas inlet 26b and an outlet 9d. A carrier system 27d is provided in the unloader chamber 25.
A description is made of a method of forming a thin film on the substrate using this device.
First, the loader chamber 22, the etching chamber 23, the deposition chamber 24 and the unloader chamber 25 are highly evacuated, respectively. Then, nitrogen gas is introduced through the nitrogen gas inlet 26a to the loader chamber 22 which is held in the evacuated state, and the pressure in the chamber is returned to below atmospheric pressure. Then, a door (not shown) is opened to introduce a plurality of silicon substrates 4 on the tray 30 to the loader chamber 22 and then the door is closed again. Next, the loader chamber 22 is evacuated through the outlet 9b by a vacuum pump (not shown) to a high degree. The lock valve 21a is opened to drive a motor of the carrier system 27a and move the silicon substrates 4 on the tray 30 to the etching chamber 23, and the lock valve 21a is closed. The operation of the lock valve 21 and the carrier system 27a is performed by switching means (not shown) which is, for example externally provided. The etching chamber 23 is held in the high vacuum state of approximately 10.sup.- 7 .about.10.sup.-8 Torr. Then, Argon gas of 10.sup.-3 .about.10.sup.-1 Torr is introduced to the etching chamber 23 through the Argon gas inlet 28a. Plasma is formed by applying a bias of several hundreds to several thousands of volts between the substrate 4 and the chamber walls by the high frequency power supply 31. Argon ions in this plasma collide with the substrate 4 which is held at a negative high voltage with high energy and sputter-etch the natural oxide film or contaminants adhering to the surface of the silicon substrate 4. After the sputter-etching is completed, the introduction of the Argon gas and the application of the bias are stopped. The Argon gas in the etching chamber 23 is evacuated and the etching chamber 23 is made to be in the high vacuum state. Then, lock valve 21b is opened to permit the motor drive of the carrier system 27bto move the silicon substrate 4 on the tray 30 to the deposition chamber 24, and then the lock valve 21b is closed. At this time, a second plurality of silicon substrates 4 is moved into the etching chamber 23. The deposition chamber 24 is also held in the high vacuum state of approximately 10.sup. -7 .about.10.sup.-8 Torr. Next, Argon gas of 10.sup.-3 .about.10.sup.-1 Torr is introduced to the deposition chamber 24 through the Argon gas inlet 28b. Then, a voltage of several hundreds to several thousands of volts is applied between the target 34 and the Argon gas by the DC power supply 35, whereby a plasma is formed. Argon ions in this plasma collide with the target 34 held at a negative high voltage with high energy to sputter the substance on the target 34. This substance is deposited on the silicon substrate 4 and the thin film is formed. After the formation of the thin film having a desired film thickness, uniformity, and quality is completed, the introduction of the Argon gas and the application of a voltage are stopped and then the deposition chamber 24 is evacuated to be in the high degree of vacuum state.
Then, the lock valve 21c is opened to permit the motor drive of the carrier system 27c to move the substrates 4 on the tray 30 to the unloader chamber 25 and then the lock valve 21c is closed. At this time, the second plurality of substrates is moved to the deposition chamber 24 and the third substrates 1 are moved to the etching chamber 23. The silicon substrates 4 etched and deposited sequentially are housed in a cassette in the unloader chamber 25. After the last silicon substrates 4 are housed in the cassette, the pressure in the unloader chamber 25 is returned to atmospheric pressure and the silicon substrates 4 having a desired thin film are taken out together with the cassette to be sent to the subsequent process station.
Since the conventional apparatus for forming a thin film by sputtering is structured as described above, it is necessary to apply a high bias between the silicon substrate 4 and the chamber walls in order to remove the natural oxide film or contaminants by sputter-etching with plasma of an inert gas such as Argon. However, there was a disadvantage in that the Argon plasma caused the silicon substrate 4 to be damaged.
The following method is also known, wherein the natural oxide film or contaminant is removed by gas etching by a high-temperature hydrogen reduction technique and then the thin films are continuously formed thereon. However, since this high-temperature hydrogen reduction technique requires a high temperature (usually above 1,000.degree. C.), thermal fusion at the PN junction is caused, making its application limited.
Japanese Patent Laying-Open No. 27,621/1986 discloses another technique for removing the natural oxide film by heating the silicon substrate at a temperature of 800.degree..about.1,000.degree. C. in the presence of a hydrogen stream. However, such a high temperature caused not only the above-mentioned thermal fusion but also caused amorphous silicon in the substrate to be turned into polysilicon with the crystallization advanced as pointed out, for example, by (E. Kinsbron, M Stenheim, and R. Knoell, Appl. Phys. Lett., Vol. 42, No. 9, 835, May 1, 1983). In view of the fact that it is desirable for polysilicon to be in the almost amorphous state with a minimum possible grain size as the substrate material in recent device manufacturing technologies, the formation of polysilicon with the advanced crystallization is not desired.
Japanese Patent Laying-Open No. 124,123/1986 discloses a process for treating the semiconductor substrate by first letting the substrate absorb a reactive gas, and then etching the substrate surface with the above reactive gas under the irradiation of light. Although this process is seemingly applicable to the removal of the natural oxide film on the semiconductor substrate, the patent fails to disclose the type of the light and reactive gas to be employed as well as the temperature at which the substrate is to be heated. If the disclosed treatment is applied in stripping the natural oxide film as it is, it is disadvantageous in that the reaction gas absorbed in the semiconductor substrate functions to etch not only the oxide film on the substrate surface but the substrate itself.